The long-term cardiac prognosis of 24 clinically healthy men with complete right bundle branch block (RBBB), identified from the 1140 men constituting the BLSA population, was assessed over a follow-up period averaging 8.4 years. When compared with a control group matched or age at which RBBB appeared, men with RBBB showed no difference in the prevalence of antecedent coronary risk factors or obstructive lung disease. Their incidence of angina pectoris, myocardial infarction, valvular heart disease, cardiomegaly, congestive heart failure, advanced heart block, or cardiac death did not differ from that of the control group over the observation period. Furthermore, at latest follow-up maximal aerobic exercise tolerance and chronotropic response to maximal exercise were not impaired in men with RBBB relative to controls. However, axis deviation leftward of-30 degrees was present in 46% of RBBB men but only 15% of controls by latest follow-up. Although a PR interval prolongation greater than 40 msec developed in only 6% of control subjects over the observation period, such prolongation occurred in 29% of men with RBBB. These results support the concept that RBBB in these asymptomatic men is a manifestation of a primary abnormality of the cardiac conduction system but has no demonstrable adverse effect.